Generally, if the aggregates are separate, the ratios are...
4 parts (20mm ) gravel
2 parts sharp sand
1 cement
Add water to taste.
A cube of concrete takes over two tonne of gear i.e. two tonnes ballast, 13-14 x 25kg bags of cement and water.
Confused by Concrete Calculations
- Thread starter EasyGoingPat
- Start date
4 parts (20mm ) gravel
2 parts sharp sand
1 cement
Add water to taste.
A cube of concrete takes over two tonne of gear i.e. two tonnes ballast, 13-14 x 25kg bags of cement and water.
Thank you all for that advice. Lot to digest, and some is contradictory so I'll have to make a judgement call, which will in part inevitably be based on cost (i.e. no point in buying stuff I don't need).
(With reference to the area of the slab - yes, this slab is indeed adding to an existing area.)
Kind wishes - Patrick
(With reference to the area of the slab - yes, this slab is indeed adding to an existing area.)
Kind wishes - Patrick
- Joined
- 7 Sep 2022
- Messages
- 4,931
- Reaction score
- 1,437
- Country
I didn't find any significant contradictions here
- Joined
- 9 Aug 2026
- Messages
- 1
- Reaction score
- 0
- Country
Hi Patrick,
For a car-bearing apron, go with a 1:2:3 (cement:sand:gravel) mix — plenty strong for driveway/car loads. Don't overthink ballast vs separate sand+gravel, ballast is just pre-mixed sand and gravel, so buying it that way is fine and usually easier (one less thing to measure on site).
For your 0.45m³:
- Cement: ~6 bags (25kg each)
- Sand: ~450kg
- Gravel: ~650kg
(or if going with ballast instead: ~1000-1050kg ballast + same 6 bags cement)
On "Handy Bags" — there's no fixed standard across merchants, most list them around 25kg but you have to check the actual product page. If your merchant does loose bulk bags (by the tonne) instead, that's usually cheaper and more predictable at this volume anyway.
Reinforcement: yes, use A142 mesh, especially since you're butting up against the existing slab — that joint is exactly where cracking tends to start without it. Fibres are a nice add-on, not a substitute for mesh.
I cross-checked these numbers using the Concrete Calculator - How Much Do I Need? | CalculationHub and they matched what I worked out by hand — worth punching in your own dimensions there before heading to Jewson, just so you've got a solid number to hold the merchant to when their bag sizes don't quite add up.
For a car-bearing apron, go with a 1:2:3 (cement:sand:gravel) mix — plenty strong for driveway/car loads. Don't overthink ballast vs separate sand+gravel, ballast is just pre-mixed sand and gravel, so buying it that way is fine and usually easier (one less thing to measure on site).
For your 0.45m³:
- Cement: ~6 bags (25kg each)
- Sand: ~450kg
- Gravel: ~650kg
(or if going with ballast instead: ~1000-1050kg ballast + same 6 bags cement)
On "Handy Bags" — there's no fixed standard across merchants, most list them around 25kg but you have to check the actual product page. If your merchant does loose bulk bags (by the tonne) instead, that's usually cheaper and more predictable at this volume anyway.
Reinforcement: yes, use A142 mesh, especially since you're butting up against the existing slab — that joint is exactly where cracking tends to start without it. Fibres are a nice add-on, not a substitute for mesh.
I cross-checked these numbers using the Concrete Calculator - How Much Do I Need? | CalculationHub and they matched what I worked out by hand — worth punching in your own dimensions there before heading to Jewson, just so you've got a solid number to hold the merchant to when their bag sizes don't quite add up.
This is an old post!
Andy
Andy
It's not over-thinking, it's just thinking! Look at some gravel, there are gaps between the stones. If you mix it with sand then a lot of that sand fills those gaps, so you end up with a mix that takes up less space than the separate gravel and sand would.
An everyday DIY concrete mix is 1 cement: 2 sand: 4 gravel (20mm). 1:2:3 is quite pastey, it needs more gravel.
1:2:4 is roughly equivalent to 1:5 using ballast, due to the sand and gravel sharing some of the space that is air when using gravel.
1:6 with ballast would be weaker than 1:2:4 with sand and gravel, as the proportion of cement would be lower.
Old and some curious thoughts too:
1100kg of sand and gravel is equivalent to 1000-1050kg of ballast?
At 1:5 as he suggests 150kg cement matches 750kg
Adding mesh won't stop the joint cracking(if anything it might make it fractionally more likely). Yo'd want to dowel the joint if this is a concern
"Solid number to hold the merchant to" WTF??
1100kg of sand and gravel is equivalent to 1000-1050kg of ballast?
At 1:5 as he suggests 150kg cement matches 750kg
Adding mesh won't stop the joint cracking(if anything it might make it fractionally more likely). Yo'd want to dowel the joint if this is a concern
"Solid number to hold the merchant to" WTF??
If we want to get needlessly nerdy then ballast comes in different pre-mixed proportions too!
I tend to just mix 1:5 and not care. But I did get some ballast that was unusually sandy and now have some very yellow concrete that looks pretty feeble. But I'm paving over it anyway so don't care.
I tend to just mix 1:5 and not care. But I did get some ballast that was unusually sandy and now have some very yellow concrete that looks pretty feeble. But I'm paving over it anyway so don't care.
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
